ClassLoader类加载器

Java自带三种“类加载器”，分别用于加载不同路径下的类文件。

System.out.println("BootClassLoader:" + System.getProperty("sun.boot.class.path"));
System.out.println("ExtClassLoader:" + System.getProperty("java.ext.dirs"));
System.out.println("AppClassLoader:" + System.getProperty("java.class.path"));

jdk1.8.0_202jrelibresources.jar;jdk1.8.0_202jrelibrt.jar;jdk1.8.0_202jrelibsunrsasign.jar;jdk1.8.0_202jrelibjsse.jar;jdk1.8.0_202jrelibjce.jar;jdk1.8.0_202jrelibcharsets.jar;jdk1.8.0_202jrelibjfr.jar;jdk1.8.0_202jreclasses

jdk1.8.0_202jrelibext;C:WindowsSunJavalibext

jdk1.8.0_202jrelibcharsets.jar;jdk1.8.0_202jrelibdeploy.jar;jdk1.8.0_202jrelibextaccess-bridge-64.jar;jdk1.8.0_202jrelibextcldrdata.jar;jdk1.8.0_202jrelibextdnsns.jar;jdk1.8.0_202jrelibextjaccess.jar;jdk1.8.0_202jrelibextjfxrt.jar;jdk1.8.0_202jrelibextlocaledata.jar;jdk1.8.0_202jrelibextnashorn.jar;jdk1.8.0_202jrelibextsunec.jar;jdk1.8.0_202jrelibextsunjce_provider.jar;jdk1.8.0_202jrelibextsunmscapi.jar;jdk1.8.0_202jrelibextsunpkcs11.jar;jdk1.8.0_202jrelibextzipfs.jar;jdk1.8.0_202jrelibjavaws.jar;jdk1.8.0_202jrelibjce.jar;jdk1.8.0_202jrelibjfr.jar;jdk1.8.0_202jrelibjfxswt.jar;jdk1.8.0_202jrelibjsse.jar;jdk1.8.0_202jrelibmanagement-agent.jar;jdk1.8.0_202jrelibplugin.jar;jdk1.8.0_202jrelibresources.jar;jdk1.8.0_202jrelibrt.jar;D:2-CodeRepocoderwwtargetclasses;D:Program FilesIntelliJ IDEA Community Edition 2021.2.2libidea_rt.jar

阅读Launcher类代码：

Launcher.ExtClassLoader var1;
try {
    var1 = Launcher.ExtClassLoader.getExtClassLoader();
} catch (IOException var10) {
    throw new InternalError("Could not create extension class loader", var10);
}
try {
    this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
} catch (IOException var9) {
    throw new InternalError("Could not create application class loader", var9);
}

ExtClassLoader是AppClassLoader的parent，他们之间不存在类的继承关系。

BootClassLoader与ExtClassLoader、AppClassLoader则没有parent或者继承关系。

阅读抽象类ClassLoader代码：

protected Class loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // 步骤1
        // First, check if the class has already been loaded
        Class c = findLoadedClass(name);
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {
                    // 步骤2
                    c = parent.loadClass(name, false);
                } else {
                    // 步骤3
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            if (c == null) {
                // 步骤4
                // If still not found, then invoke findClass in order
                // to find the class.
                long t1 = System.nanoTime();
                c = findClass(name);

步骤1：首先由当前“类加载器”查找是否已经加载过该类名；

步骤2：如果没有查找到，且parent“类加载器”非空，则由parent“类加载器”尝试加载该类；

步骤3：如果没有查找到，且parent“类加载器”为空，则由BootClassLoader“类加载器”尝试加载该类；

步骤4：如果parent“类加载器”或者BootClassLoader“类加载器”都未能加载该类，则由当前“类加载器”尝试加载；AppClassLoader和ExtClassLoader的类加载方式由共同“父类”URLClassLoader实现，具体参考findClass方法。

1.自定义类加载器构造函数为空时，parent类加载器是AppClassLoader，由父类ClassLoader的空构造函数实现，详见getSystemClassLoader；

2.自定义类加载器实现findClass方法，用来加载指定目录下的类文件，可以参考URLClassLoader的findClass实现方法；